The old saying that corn will yield well if it is "knee-high by the Fourth of July" is old and many think outdated. And truthfully, many fields that were planted in late April or early to mid-May are preparing to tassel, or have tasseled. That's more normal these days.
However, the spring was so drawn out by wet weather and the need to replant in some areas that there is still corn that is only knee-high, or barely past that stage.
How many growing degree units have accumulated since corn was planted, and how many GDDs does corn need to mature? That depends upon the location of the field in the state, the actual weather pattern this year and the relative maturity of the hybrid.
DuPont Pioneer released a chart published in an earlier Purdue University Corn & Soybean Field Guide that shows for its hybrid line-up at the time, Hybrids rated at 105 days of maturity need about 2,400 growing degree day heat units to reach maturity. This assumes normal planting date. The growing degree system takes into account growing weather whenever the temperature is 50 degrees F or above for a minimum and up to 86 degrees F as a maximum.
Hybrids at 110-day maturity need 2,400 to 2,600 GDD units. Then 115-day hybrids need 2,600 to 2,800 GDDs. Note that this is based on relative maturity in Pioneer's rating system. Other companies may use rating systems that are somewhat different in how they determine how long it takes a hybrid to reach relative maturity.
How many GDDs have accumulated? Check your local weather source for information for this year. On average, starting with March, the number of GDDs accumulated in Indiana by July 4 varies from 1,129 in northeast Indiana to 1,571 in southwest Indiana. Other numbers for other crop reporting districts are: northwest, 1,154: north-central, 1,173; east-central, 1,199; central, 1,278; west-central, 1,335; south-central, 1,470 and southeast, 1,478. These averages are based on 30-year data from 1971 through 2000.